The development of imaging systems manufactured with complementary metal-oxide-semiconductor (CMOS) technologies used to fabricate integrated circuits (ICs) has made cameras ubiquitous in high-volume consumer products including mobile computing devices and automotive products.
CMOS image sensors include an image sensor for detecting light and readout circuitry. The image sensor includes a pixel array, where each pixel converts light incident on it to an electrical signal to be read by readout circuitry. Readout circuitry on the circuit layer includes memory, an image signal processor, though-silicon vias (TSVs), and one or more analog-to-digital converters (ADCs).
Varieties of prior-art CMOS image sensors have different numbers of ADCs. Sensors with a sensor-level analog-to-digital (A/D) conversion have one ADC that receives signals from all pixels on the image sensor. Sensors with column-level A/D conversion have a dedicated ADC for each pixel column. Sensors with pixel-level A/D conversion have a dedicated ADC for each individual pixel.
In conventional CMOS image sensors, both the image sensor and the readout circuitry reside on the same layer. In stacked image sensors, the image sensor resides on a sensor layer and readout circuitry resides on a separate circuit layer positioned beneath the sensor layer. By stacking the sensor layer and the circuit layer, manufacturers may mount a given image sensor size on a smaller substrate, or fit a larger image sensor on a given substrate size.
The circuit layer of a stacked image sensor has multiple ADCs that each connects to a different pixel sub-array (PSA) formed of a contiguous subset of pixels of the pixel array of the sensor layer. By having multiple ADCs each dedicated to a respective subset of pixels, this configuration resembles non-stacked sensors with column-level A/D.